The present invention relates to a semiconducting floor covering which consists of a layer of conductive material parallel to the surface of the covering and at least one layer of polyvinyl chloride based material which is adhered to the layer of conductive material and which contains polyvinyl chloride and at least one plasticizer.
Semiconducting floor coverings are required in places where electrostatic discharges or accumulations cannot be allowed on grounds of safety. This fact concerns e.g. the explosives and computer industries, and many other places where accumulations of static electricity and discharges thereof may cause problems concerning safety or cost.
A semiconducting floor covering is understood to be material in which resistivity measured from the surface of the covering material to the grounding point is at least 50 kxcexa9 and no more than 100 Mxcexa9.
In prior art. a semiconducting floor covering is made of either rubber or plastic by adding thereto a sufficient amount of carbon black by means of which conductivity values assigned to a semiconducting covering material can be achieved. in another prior art solution the lower layer of a mat is of conductive material on top of which there is provided an isolating layer of material through which connections have been arranged made of conductive material so that the conductive material at these points extends from the surface of the coating material to the bottom. Such a solution is known e.g. from the U.S. Pat. No. 5,307,233 in which chips have been placed on a conductive bottom layer, the vertical edges of the chips having been coated by conductive material.
A problem with materials containing carbon black is that the material normally turns out black, and it cannot be covered with any colouring material. Further, the use of carbon black makes the machinery dirty during the manufacturing and causes output being wasted e.g. when changing the type of material. In addition, stretching such material in melted conditions deteriorates its conductivity. The solutions involving conductive elements of various kinds share the drawback that assembling a mat material from such elements is difficult and expensive, and furthermore, coating and measuring conductive elements for proper operation is difficult.
In addition, employing other conductive materials, such as metal particles, is expensive in such a covering, and further, the use of such additives causes significant restrictions to the colours or choosing of colours for the material because, due to the conducting properties, the coverings cannot be coloured and no surface pattern can be printed on them.
It is the object of the present invention to provide a semiconducting floor covering by means of which the prior art drawbacks may be avoided and by means of which floor coverings may be produced that work well and are wear-resistant, and which may be coloured and patterned by printing technique, and which may additionally have a colourless or transparent wear-resistant surface layer.
The floor covering according to the invention is characterized in that the plasticizer comprises 2-ethyl hexyl diphenyl phosphate and/or an aliphatic ester compound, and that the polyvinyl chloride material contains, as filler, talc and/or aluminium trihydrate so that the relative proportions of the components in the polyvinyl chloride material per 100 parts of polyvinyl chloride are 30-80 parts of said plasticizer and 1-250 parts of said filler.
The essential idea of the invention is that the principal plasticizer used for the polyvinyl chloride material (from hereafter, PVC) is 2-ethyl hexyl diphenyl phosphate (from hereafter, DPOF) and/or an aliphatic ester compound. whereby it is additionally possible to employ one or more of the following plasticizers:
monomeric or polymeric phthalates
monomeric or polymeric adipates
phosphate plasticizers
benzoate plasticizers
polyester plasticizers
trimellitate plasticizers
alkylsulphonic acid esters
polyglycoates
so that 30-80 parts of plasticizer in all have been used per 100 parts of PVC. This means that the proportion of DPOF and/or aliphatic ester compound is at least 30 parts per 100 parts of PVC, and the proportion of other plasticizer may vary. Furthermore, an essential feature of the invention is that, in addition to plasticizer. filler which contains talc or aluminum hydrate (from hereafter, ATH) has been added into the PVC material to such an extent that the proportion of pure talc or ATH per 100 parts of PVC is 1-250 parts. According to the preferred embodiment of the invention, it is possible to produce colourless or even transparent PVC whose conductivity continues to fulfill the requirements of a semiconducting material. Yet another essential idea of the invention is to have a conductive layer to which a PVC layer has been adhered so that the conductive layer is either the bottommost layer of the floor covering or advantageously close to its bottom surface.
The inventive floor covering provides the advantage that the actual floor covering may be produced by simply mixing the materials in question to each other in said proportion, after which it may be adhered to the conductive layer either to one or both sides thereof. By employing a suitable carbon-glass fiber layer, or a metal layer, the conductive layer may be arranged within the actual covering material as desired. It is further advantageous of the covering according to the invention provide that by forming e.g. two PVC layers, the lower of which being coloured, for example white, and a colourless or transparent top layer, it is possible to produce floor coverings of any desired colours, in which the top layer may further be provided with wear-resistant components without degrading its semiconducting properties.